It is an object of the present invention to provide a hot weld seam inspection system and method which detects, alerts and compensates for wedge temperature variations.
In many ultrasonic weld seam inspections with a contact mode, such as in girth weld inspections during pipeline construction, phased-array probes are used with wedges. The wedges are usually of a thermoplastic synthetic material, in particular, a cross-linked polystyrene copolymer, for example Rexolite. The wedge is placed on the part surface, e.g., pipe surface, in close proximity to the weld seam which may be still quite hot from the welding. The ultrasonic wave propagates between the wedge and the pipe surface through a liquid couplant, which in most cases is water supplied by a pump. To allow the water to flow evenly, the wedge bottom is spaced from the part surface at a small gap of about 0.1 mm. The small gap is maintained, for example, by using four anti-wearing pins screwed in the four corners of the wedge bottom to prevent it from contacting the surface. The pumped water flows through the small gap completely filling it, which enables the ultrasonic coupling.
For an inspection using a pulse-echo mode, which uses electrical pulses coming from an acquisition unit to produce excitations, ultrasonic beams of longitudinal waves (LW) generated by different apertures of the phased-array probe travel in the wedge, propagate through the small water-filled gap, penetrate into the part and then reach the weld zone. If there is a flaw in the weld zone, some ultrasonic beams may be reflected by the flaw and then return to the probe. The probe, operating as a receiver, senses the returned ultrasonic beams and outputs the flaw echo signals to the acquisition unit for signal display.
When the ultrasonic beams travel through the interface at the wedge bottom surface, some of them skip back to the wedge front, potentially causing unwanted wedge echoes. To reduce the problem, damping material is casted to the wedge front, to absorb those unwanted echoes. The solidified damping material has an acoustic impedance very similar to that of the wedge material. To efficiently absorb and scatter the wedge echoes, grooves with serrated sections are machined in the wedge front. The grooves extend approximately perpendicularly to the plane of the probe face and are machined through the wedge front height. The larger the size of the serrations, the better the efficiency of the wedge echo attenuation. However, big serrations increase the distance from the exit points to the weld. A typical sectional shape of the grooves is an isosceles triangle with, for example, 3 mm side lengths.
In practice, an inspection of a newly welded seam involves the operator making sure that the pipe surface temperature is well below the water boiling temperature, i.e., 100° C. The temperature is typically measured with a non-contact infrared temperature gauge. A pipe or part surface at a higher temperature will boil the coupling water and generate bubbles that can seriously attenuate or even cut the ultrasonic wave propagation in the coupling water layer. Preferably, the part surface temperature suitable to weld inspection should be lower than 80° C.
The pumped water flowing around and under the wedge in the small gap serves not only as an ultrasonic wave couplant but also as a coolant that keeps the wedge temperature at that of the pumped water. In other words, normally the pumped water is a perfect coolant for the wedge. On rare occasions however, when, for example, the running water is interrupted or the wedge bottom contacts directly the hot part, the wedge temperature can be affected. Unlike metal parts such as a steel part, the longitudinal wave velocity in plastic wedge is much more sensitive to temperature changes. According to Snell's Law, a LW velocity change in a wedge can induce a change of the refraction angle of the inspection beams in the part, wrongly directing the beams in the part and possibly causing a total miss of the weld zones being inspected. The requirements for temperature condition in girth weld inspections can be found in Section 9.4.3 Temperature Differentials and Control, in Standard Practice for Mechanized Ultrasonic Examination of Girth Welds Using Zonal Discrimination with Focused Search Units, Designation: E 1961-98 (Reapproved 2003)e1, ASTM International. What is even more complex is that, once the wedge is heated, the temperature field in it is normally a function of time, which makes compensation of the temperature change in the wedge by modifying the focal laws in real time very complex and difficult. Even if this method was feasible, it would be too expensive to provide it for the rare and accidental event of a wedge temperature change. Therefore the efficient way to counteract the temperature change in a wedge, is to monitor for temperature changes, and to record an alarm for the event. Then the operator can take measures to deal with the event and can continue the inspection after the temperature level in the wedge has been restored.
The following prior art addresses the subject of wedge temperature detection or wedge temperature real time compensation.
The General Electric pending patent publication US 2011/0247417 A1 discloses a method that uses wedge bottom as the reflector and use the variation of the time of flight (TOF) or the sound path from the PA probe to the wedge bottom as the indication of temperature change in wedge. The major drawback of the method is that the amplitude and TOF of the echoes from the wedge bottom can be affected by the part surface status (e.g.: when placing or lifting the search unit), possibly affecting the measurement accuracy of the TOF change.
Another inconvenience of this prior art is that the sound paths from the probe to the wedge bottom can change if the wedge bottom is worn, being potentially another factor of instability.
Yet another inconvenience of this prior art is that the zone for the detection of the temperature change is not near the weld. Because the zone of the wedge bottom with which the temperature change is monitored is below the PA probe, that zone is a little bit far from the hot weld. The separation is particularly obvious for a wedge of big angle that is often used to efficiently generate shear waves in the part.
The General Electric patent U.S. Pat. No. 8,192,075 B2 discloses another method for counterworking the temperature change in wedge. According to the method, the temperature change is sensed by two separate temperature sensors, the first one is placed on the part to measure the part surface temperature and the second one is placed on top of the wedge to measure the ambient temperature. According to the patent, the temperature field in the wedge as well as the LW velocity field in the wedge can be deduced from the temperatures measured at the two above locations, and then the focal laws are modified in real time by taking into account the LW velocity field in the wedge. This method is very expensive and very complex, and for the case of water coupling, is unable to take into account the thermal energy dissipation by the coupling water.
None of the above prior art allows directly detecting the gradient of the velocity changes in the wedge caused by the temperature changes.